High ductility, high hot tensile strength tungsten wire and method of manufacture

ABSTRACT

A method for manufacturing a high ductility and high hot tensile strength tungsten wire for incandescent lamp filaments is disclosed. The method comprises the steps of preparing a tungsten alloy, swaging a tungsten rod from the alloy, and drawing the swaged rod to wire size in multiple drawing passes. In the method, the wire is annealed between predetermined draws. It is proposed that an annealing is performed before the final drawing pass, by annealing the wire at a temperature between 1100-1300° C. 
     There is also provided a tungsten wire for incandescent lamp filament, which has high ductility and high hot tensile strength. The tungsten wire of the invention has a cold tensile strength-hot tensile strength ratio not exceeding 3.5.

BACKGROUND OF THE INVENTION

This invention relates to a high ductility and high hot tensile strengthtungsten wire for incandescent lamp filaments, and a method formanufacturing such a tungsten wire.

Lamps with an incandescent filament have been known for a long time. Inmost applications, the filaments are made of a tungsten wire, which iswound into a coil. The dimensions of the coil determine not only thelight output of the lamp, but also the optical properties of the lightbeams emerging from an optical projector system. Such projector systemsare found, among others, in headlights of automobiles or slideprojectors. Lamps with small filaments have better optical parameters,and allow the formation of a well-defined projected beam, even withsmall-sized projecting optics. Beside, projector systems not onlyrequire small filaments, but also very high lumen output.

Therefore, coils with extremely small external dimensions are beingproduced for automotive lamps and projector lamps. The small externaldimensions mean that the inner diameter of the coils is also small, inthe order of the wire diameter. The inner diameter of the coil largelycorresponds to the diameter of the mandrel, on which the filament iswound during manufacturing of the coil. The ratio of the diameter of themandrel to the wire diameter is termed as the mandrel ratio. In thismanner, coils with a small inner diameter will also have a small mandrelratio. Since the diameter of the filament wire also has a practicallower limit, filaments with small mandrel ratio are necessary for thebest possible light efficiency. Further, high light output also requireshigh filament temperatures. At high temperatures, the sagging of thefilament poses serious problems. Therefore, it is sought to manufactureso-called non-sag filaments. The non-sag ability of a filament isclosely related to the hot tensile strength of the tungsten wire fromwhich the filament is made. Hot tensile strength (hereinafter HTS) ismeasured at 1620° C., and desired values are above 0.16-24 N/mg/200 mm.

During wire production, the wire is annealed (heat treated). Thisannealing forms the mechanical properties of the wire to enable theassembly of the filaments on an automated mounting machine withoutbreakage. As mentioned above, in some instances the required opticalparameters may be obtained only with coils having a very small mandrelratio, in the order of 2 to 1.5, or even lower. This extreme mandrelratio requires that the wire remains ductile on room temperature,otherwise the wire may split or break during the winding process,particularly at those parts of the coil, which must endure the largestshaping tension or shaping stress. Ductility of the wire is closelycorrelated with its cold tensile strength (hereinafter CTS), in thesense that a wire with low CTS has a high ductility, while higher CTSvalues correspond to low ductility. CTS is measured at room temperature,and desired values for high-end, low mandrel ratio filament wires arebetween 0.5-0.7 N/mg/200 mm.

It is known in the art that the ductility of the wire may be influencedwith the annealing process. Namely, by the proper selection of times andtemperatures of the annealing in combination with the parameters of thewire drawing, the desired ductility (or the CTS) may be accomplished.However, it was noted that HTS values move in tandem with CTS values.With other words, if the annealing were directed towards increasing theductility of the wire (and thereby lowering the CTS), inevitably the HTSvalues also decreased. Conversely, when the annealing were directedtowards increased HTS values, the ductility of the wire decreased.

For example, U.S. Pat. No. 3,278,281 discloses a process formanufacturing a non-sag tungsten wire. The process involves thepreparation of a thorium-doped tungsten alloy, which is swaged andsubsequently drawn to wire size. The drawing is done in multiple drawingpasses, with multiple annealing steps between the drawing passes. Thisknown process proposes annealing after each five passes, and attemperatures of 1700° C. The resultant wire has outstanding non-sagproperties, but operates best in lamps with a relatively low efficiency,and is less suitable for high-end lamps requiring both high temperatureand high vibration resistance.

Another known process for the manufacture of a tungsten wire isdisclosed in U.S. Pat. No. 4,863,527. This process also involves theswaging of a tungsten alloy rod, and a subsequent drawing to size.During drawing, it is proposed to perform multiple annealing steps, attemperatures around 1560-1620° C. This known process results in a wirehaving a relatively low CTS, but high ductility.

The publication “The Metallurgy of Doped/Non Sag Tungsten” by E. Pinkand L. Bartha, spublished by Elsevier Applied Science, London and NewYork, 1989, further discloses that a tungsten wire need to be annealedduring drawing (see pp. 78-79), because the wire strength will increaseas the wire is drawn to smaller diameters. According to this literaturesource, the annealing will reduce the wire ductility. Depending on thefinal wire size, a combination of anneals is used to optimize theproperties of the final wire.

However, none of the known processes teach a method which would resultin a hight HTS of the wire, while reducing its CTS value. Therefore,there is a need for a method which is able to lower the CTS value of atungsten filament, and accomplishing high ductility of the wire, whilemaintaining a high HTS value of the same wire. Also, there is a need fora tungsten wire which has a low CTS/HTS ratio. There is also need for amethod which accomplishes these results without the use of anyadditional or specific tungsten wire manufacturing equipment, i. e.which does not require any radical change in exisiting manufacturingfacilities.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, there is provided a methodfor manufacturing a high ductility and high hot tensile strengthtungsten wire for incandescent lamp filaments. The method comprises thesteps of preparing a tungsten alloy, swaging a tungsten rod from thealloy, and drawing the swaged rod to wire size in multiple drawingpasses. In the method, the wire is annealed between predetermined draws.It is proposed that an annealing is performed before the final drawingpass, by annealing the wire at a temperature between 1100-1300° C.

In an embodiment of another aspect of the invention, there is alsoprovided a tungsten wire for incandescent lamp filament, which has highductility and high hot tensile strength. The tungsten wire of theinvention has a cold tensile strength-hot tensile strength ratio notexceeding 3.5.

The disclosed method may be performed with standard tungsten wiremanufacturing equipment. By performing the annealing before the lastdrawing pass, the cold tensile strength-hot tensile strength ratio ofthe wire is unexpectedly lowered, by lowering of the CTS value, andsimultaneously maintaining, in some instances even increasing the HTSvalue. Accordingly, the filaments made from the proposed tungsten wireare resistant against vibration, tolerate low mandrel ratios, andsupport high operating temperatures.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with reference to the encloseddrawings, where

FIG. 1 is a side view of an automotive lamp with a tungsten filament,

FIG. 2 is an enlarged view of a tungsten filament,

FIG. 3 is an illustrative figure explaining the concept of the mandrelratio,

FIG. 4 is a schematic illustration of a wire drawing process, and

FIG. 5 is another schematic illustration of a step in the tungsten wiremanufacturing process.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 and 2, there is shown an automotive lamp 1. Thelamp 1 has a sealed lamp envelope 2, typically made of glass. 1. Theenvelope 2 has a sealed inner volume 6 filled with a suitable gas, likeargon, krypton or xenon. The inner volume 6 contains a filament 8. Thefilament 8 is made of a tungsten wire. In certain embodiments, thefilament 8 may be single coiled, or double coiled (or coil-coiled), asshown in FIG. 2. Such coiled-coiled filaments are commonly used forhigher wattage lamps or high-end lamps. Often, the filament 8 must alsobe capable of high color temperature operation, i. e. in the heatedstate, its operating temperature may be above 2900° K., and in extremecases it may even reach 3200° K.

The filament 8 may contain an aluminum-potassium-silicon (AKS) additive,or other dopants. The dopants are added to the tungsten alloy during themanufacturing of the filament, as will be explained below.

The filament coil is formed during manufacturing by winding the wire 9of the filament 8 on a mandrel 10, as illustrated in FIG. 3. Filamentsfor high-end lamps require low mandrel ratio, in order to obtain properoptical and luminous parameters. The mandrel ratio is defined as theratio of the diameter d_(m) of the mandrel to the wire thickness d_(w),i. e. the mandrel ratio is d_(m)/d_(w) (see also FIG. 3). This requiresa wire 9 having a sufficiently high ductility, which corresponds to arelatively low CTS value, preferably as low as 0.7-0.5 N/mg/200 mm. Inthe wire manufacturing method, the ductility needed for a coiling withsmall mandrel ratio is increased by annealing the wire during the wireproduction, as will be explained below.

The wire manufacturing method starts with the preparation of a tungstenalloy, optionally comprising various additives, such as aluminum,potassium, silicon. Further additives may be selected from the group ofTh, ThO, YO, LaO, CeO, Re. The beneficial effects of such additives areknown in the art, and need not be discussed here.

Following the alloy powder preparation, the alloy powder is pressed andpresintered. The pressing and presintering is also made in a knownmanner, in order to prepare the alloy powder for the sintering.Thereafter, the alloy powder is sintered with direct current. This is aknown process step in powder metallurgy. The specific parameters of thesintering, i. e. temperature, atmosphere composition and sinteringcurrent are dependent of the geometrical and other parameters of thefurnace. Typical values of sintering current are between 3000 and 6000A, and the sintering is done in a hydrogen atmosphere. The sintering ofa tungsten alloy is also disclosed in U.S. Pat. No. 6,066,019, No.5,742,891 and No. 4,678,718.

Following sintering, a tungsten alloy wire is formed from the sinteredalloy ingot. The forming of a filament is done with known metalworkingtechniques, e. g. rolling, swaging and wire drawing. The swaging forms atungsten rod from the alloy, which is suitable for drawing to wire size.During swaging, the tungsten rod may be also annealed and/orre-crystallized. This process step is known in the art.

The swaged rod is subsequently drawn to wire size in multiple drawingpasses. As illustrated in FIG. 4, the diameter of the wire 9 decreasesas the wire 9 is forced through a series of drawing dies 11,12,13, ofwhich only three is shown in FIG. 4. (FIG. 4 is not to scale.).Typically, the wire 9 is drawn from the swaged rod to final size intwenty to forty drawing passes, depending on the final wire diameter.With this method, wire diameters between 0.3-0.04 mm are customarilyproduced. The drawing causes intensive stresses in the crystal structureof the tungsten wire, which is at least partly compensated by annealingthe wire between predetermined draws, typically after each 3-4-5 or moredrawing passes, depending on the desired result. This annealing may bedone by electric heating, or by heating with a gas burner 15, as shownin FIGS. 4 and 5. Both types of heating are known in the art.

The drawings are not made at room temperature, but the wire 9 ispre-heated during the drawing passes, typically to 500-900° C. Thedrawing tools contacting the wire 9, i.e. the drawing dies 11,12,13 canalso be heated with a suitable known heating equipment (not shown),typically to 300-400° C.

In the proposed tungsten wire manufacturing method, an annealing isperformed before the final drawing pass. During this annealing, the wireis heated to a temperature between 1100-1300° C., the actual temperatureused depending on the wire diameter. Typically, wires with a largerdiameter are annealed at a higher temperature, and thinner wires at alower temperature. As a result of this annealing just before the finaldrawing pass, the tungsten undergoes a crystal structure change thatimproves its ductility, without adversely affecting the final HTS valueof the wire. This means that the wire will maintain its good non-sagproperty, but will not break or split when wound even to small mandrelratio coils.

This step of the method is illustrated in FIG. 5, which shows theannealing being performed with a gas burner 16 before the wire 9 isforced through the die 14 during the final drawing pass, as the wire 9is drawn to final size.

In a preferred embodiment, as shown in FIG. 5, the final drawing passafter said annealing is done at a different drawing speed than theprevious drawing passes. Most preferably, the final draw is done at aslower drawing speed than the preceding draw. For example, the lastdrawing pass—as indicated by the arrow 22—may be performed at a drawingspeed approx. 65% of the speed of the last but one drawing, the latterbeing indicated by the arrow 21. Therefore, the wire 9 is changed fromone drawing line to another, as indicated by the arrow 23 in FIG. 5. Ofcourse, it is also possible to make the final drawing on the samedrawing line, though it will cause interruptions in a continuousproduction, hence it is preferable to use another drawing line for thelast drawing.

The proposed method results in a tungsten wire with outstanding non-sagand ductility properties. Due to the fact that the HTS of the wire doesnot decrease together with the decrease of the CTS value, it is possibleto manufacture tungsten wires having a cold tensile strength-hot tensilestrength ratio not exceeding 3.5.

For example, with a 240 mg/200 mm size tungsten wire hot tensilestrength values of 0.16 N/mg/200 mm were accomplished. For the samewire, a cold tensile strength value of 0.52 N/mg/200 mm was accomplishedresulting in a CTS/HTS ratio of 3.25.

For another wire with a 5.2 mg/200 mm size, hot tensile strength valuesof 0.210 N/mg/200 mm were accomplished. For the same wire, a coldtensile strength value of 0.745 N/mg/200 mm was accomplished, resultingin a CTS/HTS ratio of 3.43. Such thin and ductile wires are well suitedfor small mandrel ratio coils.

Some illustrative CTS and HTS values obtained with the method are listedin the table below:

TABLE I Wire Size Decrease in mg/200 CTSN/mg/ HTSN/mg/ CTS/ CTS/HTS mmTechnology 200 mm 200 mm HTS ratio, % 5.17 Prior art 0.960 0.217 4.425.17 Annealed * 0.745 0.210 3.43 23 41.60 Prior art 0.723 0.1600 4.5241.60 Annealed * 0.607 0.1770 3.43 25 77.60 Prior art 0.610 0.1550 3.9477.60 Annealed * 0.570 0.1700 3.35 15 240.00 Prior art 0.551 0.1740 3.75240.00 Annealed * 0.520 0.16.00 3.25 14 Annealed * = Annealed before thefinal drawing pass

The proposed type of tungsten wire is applicable for all types of lamps,and it is principally recommended for the production of special high-endand automotive lamps with double spiral filaments of small mandrelratio. A classical example is a 24 V, 21 W stop lamp for automobiles,which is subjected to a high number of switch on-switch off cycles,beside the intensive vibration. The application of this wire willlargely reduce the breakage or deterioration of the filaments duringmanufacture of the coils, and also increases the lifetime of the lamps.

With the suggested method, the general mechanical properties of thefilaments of special incandescent lamps with small mandrel ratio areimproved, while it is still possible to produce both the wire and thefilaments with standard manufacturing equipment. This means in practicethat the production facilities for traditional K, Si, Al doped tungstenwire may be used, while decreasing defect rate of the filaments duringproduction and use. The improved ductility of the wire will result insuperior filament winding quality. The wire retains its desired fibrousstructure, which is essential for long-life, non-sag filaments.

The invention is not limited to the shown and disclosed embodiments, butother elements, improvements and variations are also within the scope ofthe invention. For example, it is clear for those skilled in the artthat beside the annealing step before the last drawing pass, a number offurther annealing steps may be performed during the various drawingpasses, in combination with re-crystallization or similar heattreatments.

1. A method for manufacturing a high ductility and high hot tensilestrength tungsten wire for incandescent lamp filaments, comprising thesteps of preparing a tungsten alloy, swaging a tungsten rod from thealloy, drawing the swaged rod to wire size in multiple drawing passes,annealing the wire between predetermined draws, in which an annealing isperformed before the final drawing pass, by annealing the wire at atemperature between 1100-1300° C.
 2. The method of claim 1, in which thefinal drawing pass after said annealing is done at a different drawingspeed than the previous drawing passes.
 3. The method of claim 2, inwhich the final drawing pass after said annealing is done at a slowerdrawing speed than the previous drawing passes.
 4. The method of claim3, in which the final drawing pass after said annealing is done at adrawing speed substantially 0.65 times the drawing speed of the previousdrawing pass.
 5. The method of claim 1, in which the wire is drawn fromthe swaged rod to final size in twenty to forty drawing passes.
 6. Themethod of claim 1, in which the wire is pre-heated during the drawingpasses.
 7. The method of claim 6, in which the wire is pre-heated to500-900° C. during the drawing passes.
 8. The method of claim 1, inwhich the drawing tools are pre-heated during the drawing passes.
 9. Themethod of claim 8, in which the drawing tools are pre-heated to 300-400°C. during the drawing passes.
 10. The method of claim 1, in which thewire is further annealed between drawing passes preceding the finaldrawing pass.
 11. A tungsten wire for incandescent lamp filament, havinghigh ductility and high hot tensile strength, having a cold tensilestrength-hot tensile strength ratio not exceeding 3.5.
 12. The wire ofclaim 11, having a hot tensile strength between 0.16-0.24 N/mg/200 mm,measured at 1620° C.
 13. The wire of claim 11, having a cold tensilestrength between 0.50-0.75 N/mg/200 mm, measured at room temperature.14. The wire of claim 11, being formed as a coil, and having a mandrelratio not exceeding
 2. 15. The wire of claim 11, comprising additivesselected from the group of Al, K, Si.
 16. The wire of claim 11,comprising additives selected from the group of Th, ThO, YO, LaO, CeO,Re.